Electromigration is the movement of atoms through a metal conductor (such as aluminum, for example) in response to the passage of current through the conductor. Over a period of time, electromigration causes voids (absences of metal) to form in the metal conductor. In a semiconductor integrated circuit, these voids can ultimately grow large enough to form an open circuit in the conductor and cause it, and the integrated circuit, to fail. Therefore, electromigration is a known "wearout" characteristic of a semiconductor device.
However, if a metal conductor can be fabricated with a strong (111) crystalline orientation or "texture," that conductor will be more resistant to the effects of electromigration. Specifically, a strong (111) texture in a metal conductor will reduce the motion (migration) of atoms as a current flows through the conductor.
Standard sputtering techniques have been optimized to produce strong (111) textures in metallization layers in semiconductor devices with tungsten deposited by chemical vapor deposition to fill both vias between metallization layers and contacts to junction or gate. Aluminum alloy or copper alloy, however, is preferred over tungsten in order to increase the speed of a semiconductor device and reduce the cost of processing. Unfortunately, standard sputtering techniques do not provide the necessary step coverage to fill vias or contacts using aluminum alloy or copper alloy. Consequently, for aluminum alloy or copper alloy, other contact forming techniques have been used, such as high temperature reflow (e.g., at temperatures greater than 500.degree. C.), high pressure extrusion (e.g., at pressures greater than 60 M Pa), or chemical vapor deposition process.
In chemical vapor deposition processes and high pressure via/contact filling techniques for aluminum alloy or copper alloy, metal interconnects are formed by sequentially depositing a layer of titanium, a layer of titanium nitride, and a layer of metal within a via or contact, on an oxide layer. The layer of titanium functioned to reduce electrical resistance in the via or contact. The titanium nitride layer functioned to prevent the layer of titanium from reacting with the metal layer. Otherwise, the reaction would have transformed the metal layer into a metallic compound having highly resistive properties, thereby reducing the effectiveness of the metal layer as a conductor. Typically, the titanium nitride layer is deposited by chemical vapor deposition or long-throw sputtering geometry process to improve the step coverage. However, these two methods of depositing titanium nitride produced a titanium nitride layer with poor (111) texture, thus resulting in a weak (111) crystalline structure for any metal layer deposited over the titanium nitride layer. Consequently, any metal interconnects that were part of the metal layer were unreliable because of electromigration effects.